Conventionally, monolithic integrated circuit dies can be manufactured with two transistor gate oxide thicknesses. A thick gate oxide is commonly used for transistors in circuits for input to and output from an integrated circuit die (I/O devices) and a thinner gate oxide is used for all other transistors on the die (functional devices). Although it is possible to select between varying thicknesses for the thinner gate oxide layer depending on the desired performance and power characteristics of the circuit to be implemented, until recently functional devices were commonly limited to a single gate oxide thickness. For example, thinner gate oxides enable higher frequency operation and hence higher performance at a cost of higher leakage current. Thicker gate oxides provide lower leakage current but sacrifice higher frequency operation. With the advent of triple gate oxide (TGO) manufacturing processes, it is now possible to have three transistor gate oxide thicknesses with varying performance characteristics on a monolithic integrated circuit die. Accordingly, there is a need in the art to utilize the TGO process to produce integrated circuits in order to advantageously utilize the varying performance characteristics enabled by the TGO process.
Gate oxide thickness is commonly described in “equivalent physical oxide thickness” terms because current processes do not necessarily use pure silicon to create the gate oxide. Some processes employ a dielectric which has a higher dielectric constant than silicon. Such processes report the thickness of pure silicon required to achieve equivalent capacitance with the dielectric actually used. In current processes, equivalent physical oxide thicknesses can commonly vary between approximately 3-6 nm for I/O devices and between approximately 1-2 nm for functional devices. FIG. 1 illustrates a cross-sectional view of a conventional CMOS transistor, and in particular the location of the gate oxide. Herein all references to gate oxide thickness also apply to equivalent physical oxide thickness.
Integrated circuits (ICs) are generally thought of as being composed of interoperable blocks or functional units (sometimes called cores) of circuitry which perform some particular function and cooperate in order to function as a complete IC. For example, processors or processor cores are integrated circuits designed to perform a particular set of computational functions. A common method of achieving greater computational performance in an IC is to employ a plurality of processor cores. The processor cores in such a multiple-core systems may be identical or may have differing architectures, power consumption and performance capabilities that make them suitable for particular kinds of tasks. Examples of combinations include but are not limited to: (1) identical processors operated at differing voltages and frequencies; (2) processors which are designed with different sets of functions (for example, one fast processor with an comprehensive instruction set and one slow but power-efficient processor with a reduced instruction set); and (3) identical processors manufactured with different processes leading to different performance and power characteristics.